Does msync() write to file only changed pages or wholly cached buffer?

Question

I use a POSIX-function mmap() in Linux. But when I do msync(), then does it write cached buffer to a file wholly, or somehow it is noted somewhere which pages have changed, and which did not, and writes to the file only changed - i.e. does msync() write to file only changed pages or cached buffer wholly?

Suppose if we work with file of 1 GB by using mmap(), read it all, ie through a lot of page-fault raised in memory of the entire file. Then we changed only one byte, and called msync(), then it will begin to record the whole 1GB to a file or somehow determine the page you want changed and will only keep it, and how it is defined - how does it do this??

Answer 1

msync does what you want: it will only write pages which have actually been modified. Actually, msync is largely a no-op on Linux or any system with a proper virtual memory and page cache system; read will immediately see anything written to the mmapped pages, even without msync. It's largely an analog of fsync, but with a memory address range rather than a file descriptor as its argument. One other observable effect is that it causes the file modification time to be updated.